Description

Summary:
This change to the valid hole fits adds built-in syntax candidates (like (:) and []),
so that they are checked in addition to what is in scope.

The rest is merely a refactor and export of the functions used to find the valid
hole fits, since there was interest at ICFP to use the valid hole fit machinery for
additional uses. By exporting the tcFilterHoleFits function, this can now be done
without having to rely on parsing the actual error message.